Mechanism of heavy chain class switch

Abstract

During B cell development, successive DNA rearrangements assemble complete heavy and light chain antibody genes from component sequences present in multiple copies in the germline. Productive heavy chain variable region assembly results in synthesis of IgM. Subsequently, class switch recombination may shift the heavy chain variable region to a site 5{dollar}\sp\prime{dollar} to another heavy chain gene, leading to the production of a new antibody class with different functions within the immune system.;The first set of experiments addresses the role of specific DNA sequences in the switch process. Upstream of each heavy chain gene is a region of tandemly repeated DNA which some have proposed to be a substrate for this recombination. A primary class switch has two breakpoints: a 5{dollar}\sp\prime{dollar} site upstream of the IgM constant region gene and a 3{dollar}\sp\prime{dollar} site upstream of the new isotype. Both switch sites were mapped in a panel of hybridomas. While the 3{dollar}\sp\prime{dollar} switch sites are within tandem repeats, the 5{dollar}\sp\prime{dollar} breaks fall outside of, and are clustered immediately 5{dollar}\sp\prime{dollar} to, the {dollar}\mu{dollar}-associated repeats, which are thus deleted in cells that have undergone switch recombination. Loss of these tandem repeats appears to be an ongoing process in normal B cells and may be a prerequisite for switching. This suggests that the {dollar}\mu{dollar}-associated tandem repeats play a regulatory role in the switch process; their presence would maintain IgM expression while their loss would permit switching.;Two models for class switching have been proposed. Class switch recombination may follow a looping out and deletion process, similar to that documented for V-region assembly, or proceed by unequal sister chromatid exchange (USCE). The mouse myeloma MPC 11 has duplicated its IgG2a heavy chain gene through USCE. DNA sequence analysis of the region of crossover reveals that it falls within a region of TC and TG dinucleotide repeats. The TC repeats appear to be involved in the USCE and are, in fact, amplified at the recombination site. This is the first molecular description of any USCE event; however, the relevance of these observations to class switching and other USCE events remains to be determined.